The present invention is a nondestructive method for evaluating the quality and integrity of a thermoplastic weld having an embedded susceptor. The method uses an impulse coil to vibrate the susceptor and an acoustic sensor to listen to the vibration to assess the weld quality, generally through analysis of the return signal in the frequency domain.
Composite materials lend themselves to bonded structures better than to fastened ones. Bonded composites have received limited use in critical aerospace structures, however, because the bonds can vary in strength or stiffness even if they have no discrete bond line defects (disbonds, porosity, voids, cracking, etc.). Traditional nondestructive inspection methods rely upon quantifying these defects to predict the flightworthiness of the structure, but are unable to ascertain the cohesiveness of the bond at any location if defects are absent. Nondestructive identification of low strength bonds and regions of xe2x80x9ckissing unbondsxe2x80x9d (bonds of near zero strength) remains a significant goal solved only in a few specific bonded applications where the results of shear or tensile tests have been correlated to a particular NDE signal feature. Modified pulse-echo ultrasonic testing (UT) has been successful in finding the discrete defects (voids, delaminations, porosity), but not xe2x80x9ckissing unbondsxe2x80x9d and low strength bonds. Infrared thermography, shearography, eddy current, and various high and low frequency ultrasonic methods have also been unsuccessful in discerning bond quality in thermoplastic welds.
The present invention provides a nondestructive method for testing bond quality using an electromagnetic (EM) pulse to induce vibrations in the embedded susceptor and an acoustic receiver to listen to and to record the induced vibrations. Analysis of the received vibration signal discriminates bond quality.
The present invention inspects bond lines that contain conductive material, especially those formed using a copper mesh susceptor, using high energy electromagnetic pulsing with acoustic receiving in a single inspection head to produce a unique evaluation technique. The inspection head contacts the outer skin of the structure whose bond is being evaluated, and contains a pancake type copper coil containing windings with rectangular cross sections designed to effectively couple to the susceptor. The closing of a switch releases a charge built up in a capacitor bank, which creates a high energy pulse in the coil. The electromagnetic field produced by the current pulse couples to the susceptor, opposes the driving magnetic field of the coil, and creates a force on the susceptor. With the help of a finite element code for electromagnetic interactions, we have been able to model the test setup and predict the forces on the susceptor. For the EM pulse produced by the discharge of a capacitor bank (proportional to the voltage of the coil), the normal stresses induced in the susceptor are shown in FIG. 3. The frequency of the stress peaks in the time domain is twice that of the voltage peaks at the coil, because a change (positive or negative) occurs on both sides of each peak.
The stresses incite vibrational modes in the susceptor and surrounding composite, creating an acoustic wave that we receive and record with an electromagnetically shielded acoustic-emission (AE) transducer at the center or around the outside of the coil (FIG. 4). While the size of the signal will depend upon the size of the incoming pulse, the susceptor conductivity, and the depth of the bond line, frequency-related features of the signal can be correlated to bond quality. We have demonstrated experimentally that a susceptor that is not fully bonded to the surrounding substructure will produce low frequency modes that are absent in a well bonded structure. The difference in the frequency response produced from a good bond and a poor bond is shown in FIGS. 5 and 6, respectively. FIGS. 5 and 6 plot Fourier transforms from the time domain to the frequency domain of the ultrasonic signal received at the AE transducer.
Bonds on both sides of the susceptor can be separately examined. These bonds experience both tensile and compressive stress when the susceptor vibrates. As the vibration occurs, the bond above the susceptor will be under tension as the bond below is under compression, and visa versa. We measure the response of the bond to a given induced stress level, permitting a type of in-situ proof-testing of the bond quality. xe2x80x9cKissing unbondsxe2x80x9d or low strength bonds can be identified with lower energy pulses.
Some radomes contain conductive layers (the FSS layers), which may be inspected with this device. In addition, a conductive layer can be added to adhesive bond lines to produce an inspectable bonded structure from an otherwise uninspectable one.